The catalyst for olefin polymerization may be classified into Zeigler-Natta catalyst system and metallocene catalyst system, and these two highly active catalyst systems have been respectively developed according to their characteristics. Zeigler-Natta catalyst had been widely used in prior commercial processes since it was invented in 1950's, but it was characterized in that the molecular weight distribution of the polymers obtained by using the same was wide because it is a multi-site catalyst having a plurality of active sites and thus there was a problem of that the composition distribution of the comonomers in the polymer was not even and there was a limitation in securing the properties required.
Metallocene catalyst consists of a combination of a main catalyst in which a transition metal compound is a main component and an organometal compound, a cocatalyst, in which aluminum is a main component. Such catalyst is a homogeneous complex catalyst and a single site catalyst. The polymer of which the molecular weight distribution is narrow and the composition distribution of the comonomers is even can be obtained by using the same due to the single site characteristic, and the stereoregularity, the copolymerization characteristics, the molecular weight, the crystallinity, and so on of the polymer can be changed by varying the ligand structure of the catalyst and the polymerization conditions.
Meanwhile, an ansa-metallocene compound is an organometal compound having two ligands which are connected by a bridge group, wherein the rotation of the ligands is prevented and the activity and the structure of the metal center are determined by the bridge group.
Such ansa-metallocene compound is being used as a catalyst for preparing an olefinic homopolymer or copolymer. Particularly, it is known that a high molecular weight polyethylene can be prepared and the microstructure of a polypropylene can be controlled by using the ansa-metallocene compound including a cyclopentadienyl-fluorenyl ligand. Furthermore, it is known that the ansa-metallocene compound having an indenyl ligand has excellent activity and can be used to prepare a polyolefin having an enhanced stereoregularity.
The present inventors have disclosed an ansa-metallocene compound of novel structure which can provide various selectivity and activity to polyolefin copolymers in Korean Patent Publication No. 10-2013-0125311.
Meanwhile, when the ansa-metallocene catalyst has bis-indenyl ligand, two types of isomer, namely, a racemate and a mirror-symmetric meso diastereomer, may be formed according to steric arrangement of two ligands. The racemate is preferred because it is used to prepare an isotactic polymer having high crystallinity and melting point and large specific gravity and mechanical strength, but the mirror-symmetric meso diastereomer is avoided because it prepares an atactic polymer. However, the racemate and the mirror-symmetric meso diastereomer are simultaneously prepared in the preparation process of the ansa-metallocene catalyst, and thus the structure of the ansa-metallocene catalyst in which an excess of the racemate can be formed must be importantly considered. Furthermore, the ansa-metallocene catalyst which can prepare a polyolefin having higher molecular weight is needed.
Therefore, the present inventors have conducted various studies on the ansa-metallocene catalyst that has higher activity than prior known catalysts and can prepare a high molecular weight polyolefin, and accomplished the present invention by recognizing that the metallocene catalyst having the structure of the present invention disclosed in this specification satisfies above characteristics.